1. Technical Field
The present invention relates to a quartz glass optical communication fiber (referred to hereinafter as an “optical fiber”) and to a method of manufacturing an optical fiber preform thereof.
2. Related Art
Generally, an optical fiber includes a core portion that transmits light and a cladding portion that surrounds the core portion. The core portion generally has a higher refractive index than the cladding portion. The optical fiber is obtained by heating and softening an optical fiber base material in an electric furnace and then drawing the optical fiber base material to have the desired thickness. The optical fiber base material is manufactured by forming a core rod including the core portion and, in some cases, part of the cladding portion, and then applying another cladding portion on the outside of the core rod. To form the core portion, VAD (vapor-phase axial deposition), OVD (outside vapor deposition) MCVD (modified chemical vapor deposition), or PCVD (plasma chemical vapor deposition) is used. The cladding portion applied to the outside of the core rod may be directly deposited on the core rod using OVD and then vitrified to form a transparent glass in a heating furnace, or may be formed by covering the core rod with an independently formed cylinder.
The method of covering the core rod with the cylinder is referred to as RIT (rod in tube) or RIC (rod in cylinder). Here, the structure resulting from the manufactured cylinder being drawn to have the desired thickness is referred to as a “tube,” and the manufactured cylinder itself is referred to as the “cylinder.” In other words, the size of the cylinder is greater in the RIC method than in the RIT method, but both methods are the same from a technical view point, and the distinction therebetween is vague. Accordingly, the method of providing the cladding portion by covering the core rod with a cylinder is referred to generally hereinafter as the RIT method.
With the RIT method, the core rod is inserted into the cylinder and heated to be integrated as a single body, and the drawing to the desired thickness may be performed simultaneously while forming the cylinder and the core rod as a single body, or the cylinder and the core rod may be integrated as a single body without simultaneous drawing. Obviously, performing the integration without drawing is simpler. On the other hand, although performing the drawing simultaneously with the integration is more complicated, this method has the benefit that the preform can be manufactured to have a size corresponding to the draw furnace into which the preform will later be inserted. In this case, the original cylinder and core rod can be larger without being limited by the size of the draw furnace, thereby achieving a higher productivity rate than the method of only performing the integration. It should be noted that the drawing to form the optical fiber can also be performed at the same time as the integration.
Here, OVD is used to manufacture the composite quartz cylinder. The following is an example of this manufacturing method. An aluminum core rod is used as a substrate, a silicon compound such as silicon tetrachloride is used as the raw material, and silicon dioxide fine particles formed by hydrolyzing the silicon compound in an oxygen flame are deposited on the substrate to form a soot deposition body. After the deposition is finished, the aluminum core rod is removed, a carbon core rod is inserted, the resulting structure is dehydrated in a chlorine atmosphere, and the structure is then vitrified in a heating furnace to form clear glass. The dehydration and vitrification may be performed in series in the same furnace, or may be performed in different furnaces. When vitrification is performed with a constant normal pressure, it is preferable to use an atmosphere containing a gas with low molecular weight, such as helium, because this will reduce the air bubbles remaining in the glass. If a gas with high molecular weight, such as nitrogen, is used, the heating and vitrification are preferably performed under reduced pressure, because this will reduce the air bubbles remaining in the glass.
Next, the carbon core rod is removed to obtain the quartz glass cylinder, and in this state, the inner and outer surfaces of the cylinder are both rough. The inner and outer surfaces are ground and polished, so that the inner diameter and outer diameter of the cylinder are uniform. The grinding is a process for eliminating large amounts of unevenness, and the polishing is a process performed after the grinding to remove the remaining small amounts of unevenness. After sufficient polishing, the surfaces become mirrors. The polishing is usually achieved by using a polishing cloth to rub a surface while passing a liquid including pure water and cutting fluid over the surface to increase the polishing efficiency. The polishing cloth may have diamond abrasive grains provided thereon, for example.
The quartz glass cylinder obtained in the manner described above is integrated with the core rod using the RIT method to form the preform, which is then drawn to form the optical fiber. Transmission loss is one important characteristic of the optical fiber. Generally, in a single mode optical fiber, the core portion is doped with Ge and the refractive index thereof is approximately 0.35% higher than that of the cladding portion, which is often made of pure quartz. In such an optical fiber, the minimum transmission loss occurs near 1550 nm, and is in a range approximately from 0.180 to 0.190 dB/km. The transmission loss at 1550 nm and at longer wavelength regions changes at the interface between the core rod and the cladding portion applied later.
If there are impurities near the interface, the transmission loss in longer wavelength regions increases. This can be thought of as being caused by absorption loss due to the impurities or microbending loss due to small diameter changes in the core caused by the impurities. In order to avoid this problem, the preform is manufactured using the RIT technique after washing the inner surface of the cylinder with an HF solution and pure water. Washing with the HF solution removes a portion of the glass along with impurities on the surface, as a result of the etching function, and this roughens the surface that is washed. The surface roughened in this way causes an increase in the number of air bubbles at the interface during manufacturing of the preform using the RIT technique, and is therefore undesirable.